Pulsed lasers currently exist, the pulses of which use intra-cavity switches, known as Q-switches, where Q represents the quality factor of the resonant cavity. They are known as Q-switched lasers.
There are two phases in a Q-switched laser. The pumping phase enables the storage of the pumping energy in the laser material. The closed cavity switch prevents resonance. The switch is opened to generate a pulse. Resonance is possible. The light present in the modes of the cavity is amplified by the laser material. An intense pulse is formed. The energy of this pulse is proportional to the energy stored in the laser material during pumping. The switching must be fast in order to ensure effective control of the energy and temporal profile of the pulse. Conventionally, only the very fast opening of the Q-switch is controlled. The controlled closure shortly after the opening may allow the energy per pulse to be reduced.
Each laser material transition has a lifetime. This is the time, excluding pumping, required for half of the population in an excited state of the laser transition to disappear. It is also the time required in order to attain half of the population in an excited state for a very long pumping duration in the absence of any parasitic effect that may reduce pumping efficiency.
If the period between the pulses is long compared with the lifetime of the excited state of the laser transition used, the energy per pulse is the maximum. Any increase in the period does not modify the energy per pulse. For a given laser, this energy per pulse is controlled by the power of the pumping and the lifetime of the excited state.
This is illustrated by FIG. 1 which shows two examples of gain curves as a function of time, for a lifetime of the excited state of 250 μs, this gain expressed in arbitrary units being accumulated in the amplifying medium of a Q-switched laser, by a continuous pumping A for the curve “a” and A/5 for the curve “b”. For periods greater than 830 μs, the available gain varies by less than 10%; the energy per pulse varies in similar proportions.
At the other end of the curve, when the period is reduced, the energy per pulse is no longer controlled by the lifetime of the excited state. The laser operates at medium power. With a fixed period, the energy per pulse is proportional to the period separating each pulse. Any change in the period modifies the energy per pulse; the latter depends on the energy of the preceding pulse and the period separating them. In the examples shown in FIG. 1, this is the domain of periods less than 250 μs.
Finally, since the energy per pulse varies in proportions similar to the gain, for pulse periods varying between 150 and 1000 μs highly disparate energies per pulse are obtained, since they vary between 150 and 500 arbitrary units.
Moreover, the pulses must meet a minimum energy requirement for performance, but must not exceed a given energy threshold in order to avoid irreversible degradation of the laser.
A plurality of methods are used to obtain similar pulses with a variable repetition frequency.
The first solution carries out a sorting at the output of the Q-switched laser. The laser has a fixed repetition period referred to as the base period. The output pulses are either rejected or transmitted. The periods obtained are therefore limited to multiples of the base period; the starting position of each pulse train is imposed.
A different solution consists in modulating the continuous pumping source power as a function of the required time for the emission of each pulse. The modulation of the pumping power compensates for the effect of the increase in the energy of the pulse with the pumping duration; but this modulation is possible only insofar as the response time constant of the pumping source allows it, notably if the pumping source is a laser. It is not always possible to modulate the pump quickly enough, or it is not possible to predict sufficiently in advance when the following pulse will have to be emitted. The variable power of the pumping modifies the thermal equilibrium point of the resonant cavity when the repetition frequency changes, generating a thermal instability within the Q-switched laser.
A different solution involves the control of the time and duration of the opening of the Q-switch. The duration of opening depends on the energy of the preceding pulse and the elapsed time. The duration of opening is therefore controlled as a function of the time that has elapsed since the preceding pulse and of its energy. The opening and closing switch durations must also be controlled. A sophisticated electronic control system is necessary to open and close the Q-switch precisely. This double control is unusual and complex and its adjustment is difficult. For periods changing with each pulse, the control laws are difficult to adjust and readjustments are necessary during the life of the laser. Complexity reduces operating safety and reliability.